I've long thought that the idea of reading The Chronicles of Narnia in chronological order is plain silly. The Lion, the Witch and the Wardrobe was the first, and needs to be read first. Prequels, like The Magician's Nephew, demand a knowledge of the "quels" they're "pre-" to. As I've been reading the series with my kids over the last few months (we step into The Last Battle today), I've become even more convinced that Lewis told them in the right order when he wrote them, not when he talked about it later.
John Miller over at National Review Online makes the case wonderfully. Mostly by quoting the two books vying for first read. He's right, it's Wardrobe all the way.
Lewis put the matter more succinctly in a letter toward the end of his life: "An author doesn't necessarily understand the meaning of his own story better than anyone else."
Lewis of course understood the meaning of Narnia. But a wise expert is not the same thing as a final authority — and on the question of which Narnia book should come first, Lewis was utterly wrong.